Chapter 12 Work and Machine

12.3 Simple Machines NOTES

There are 6 basic kinds of simple machines:

1. the inclined plane2. the wedge3. the screw4. the lever5. the wheel and axle6. the pulley

Nothing fancy here. See if you can say them all.

1) The Inclined Plane : a flat, sloped surface

This allows you to exert your input force over a longer distance.

input force = force needed to push or pull an object

output force= force you would need to lift the object without the inclined plane ( the weight of the object)

length of incline ideal mechanical advantage = height of incline

The longer the incline, the less input force you need to push or pull an object.

2ft/1ft = 2 times input force

3ft/1ft = 3 times input force

4ft/1ft = 4 times input force

2) Wedge:

*a device that is thick at one end and tapers to a thin edge at the other end

* an inclined plane that can move

With this, instead of moving an object along the inclined plane, you move the inclined plane itself.

length of the wedgeideal mechanical advantage = width of the wedge

The longer and thinner a wedge is, the greater its mechanical advantage. That’s why sharp knives cut better than dull ones.

3) Screws:• an inclined plane wrapped around a cylinder

• this spiral inclined plane forms the threads of the screw

1. You exert input force on the screw.2. The threads of the screw act like an inclined plane to

increase the distance over which you exert the input force.3. As the threads turn, they exert an output force on the wood,

pulling the screw into the wood.4. Friction between the screw and the wood holds the screw in

place.

The closer together the threads of the screw are, the greater the mechanical advantage.

The closer the threads, the more times you must turn the screw. Your input force is applied over a longer distance which results in an increased output force.

Length around the screws is the length of the inclined plane. The length of the screw is the height.

length around threadsideal mechanical advantage = length of the screw

4) Levers: • rigid bar that is free to pivot, or rotate, on a fixed

position – the fulcrum

1. increases input force2. changes the direction of

the input force

Ideal mechanical advantage = distance from fulcrum to input force distance from fulcrum to output force

Levers are classified according to the location of the fulcrum relative to the input and output forces.

First-Class Levers: • always changes the direction of the input force• If the fulcrum is closer to the output force, they

increase force.• If the fulcrum is closer to the input force, they

increase distance.

Second-Class Levers:• increase force• DO NOT change the direction

Third-Class Levers:• increase distance• DO NOT change the direction of the input force

FULCRUM POSITIONING

5) Wheel and Axle:• made of 2 circular or cylindrical objects fastened

together that rotate about a common axis

The object with the larger radius = wheelThe object with the smaller radius = axle

wheel

axle

If the wheel (handle) is larger than the axle (shaft), the axle rotates and exerts a large output force.

The wheel and axle increase your force, but you must exert your force over a long distance.

When the input is applied to the axle, the input force is exerted over a short distance. Therefore, the wheel and axle multiplies distance.

radius of wheelmechanical advantage = radius of axle

6) Pulley: • made of a grooved wheel with a rope or cable

wrapped around it

Pull on one end of the rope = input force

The other end of the rope the output force pulls up on the object you want to move.

A pulley can make work easier in 2 ways:

1) It can decrease the amount of input force needed to lift the objects.2) The pulley can change the direction of your input force.

There are 2 basic types of pulleys:

FIXED PULLEY: attach this to a structure

MOVABLE PULLEY: attach the object you want to move

The ideal mechanical advantage of a pulley is equal to the number of sections of rope that support the object.

Look at p. 431. Discuss different pulleys, including block and tackle.

Most of the machines in your body are levers that consist of bone and muscle.

Your muscles are attached to your bones by connecting tissue called tendons.

The tendons and muscles pull on bones, making them work as levers.

The joint, near where the tendon is attached to the bone, acts as the fulcrum.

The muscles produce the input force.

The output force is used for doing work.

Your front teeth, incisors, are shaped like wedges to enable you to bite off pieces of food.

Turn to page 432 and look at the first-, second-, and third-class levers used in our bodies.

A compound machine is a machine that utilizes two or more simple machines.

The ideal mechanical advantage of a compound machine is the product of the individual ideal mechanical advantages of the simple machines that make it up.